What’s your full name?
Where are you?
What month is it?
What day of the week is it?

Walter Keller tried to speak, but no words came out, only a dry rasp. The man asking the questions had dark, close-cropped gray hair and a kind, level gaze.

Eventually the man left the room. Walt wriggled up in his bed. Someone put a hand on his shoulder and pressed him gently back into the mattress.

Walt—tall and rawboned, with marbly green eyes and muscles hardened by a lifetime of physical labor—tried to elevate himself. An earsplitting noise went off. A nurse came running in and told him to get back down. When she left, Walt found that the nurses had clipped an alarm to his bed that would alert them whenever he tried to get up. He ripped it off and threw it to the ground.

The man was back:

What’s your full name?
Where are you?
What month is it?
What day of the week is it?

Walt had to get out of this room. He had a baseball game to coach, over at the ball field in Upland, California. The Upland Pony Giants were waiting for him. Michael was waiting, the skinny kid with the cannon arm, and Cody, the kid who could steal two bases on one pitch. The game was starting in five minutes—didn’t anyone understand that?

Walt glanced to his side and saw his 19-year-old son, Dustin. Dustin was here, thank God. Dustin would listen.

“Dustin, get my shoes,” Walt croaked. “Dustin, I have to get out of here. I’ll give you a ride on the boat.”

Walt knew his boat was right outside the door of the room—the wakeboard boat he drove every year up and down Lake Mohave in Nevada, giving water-ski rides to his grandkids. His boat was right here at the hospital. If he could only make it out of the bed, to the door, he could climb into the boat and drive it back to his house.

Walt wasn’t in California, as he thought. He was 2,700 miles east, in Philadelphia, where he’d come to be a guinea pig in a test of a new kind of cancer treatment. Leukemia had invaded his bone marrow and spread like a stain through his lymph nodes; the traditional options, including chemo and radiation, had failed. He was 58, and his body groaned with tumors potentially weighing as much as seven pounds. Walt needed something radically different if he was going to live. And the treatment he’d been given a few days ago was certainly that.

Over the past several years, a couple of hundred mice had received it, but Walt was only the seventh adult human. (Six men had preceded him, as well as a six-year-old girl.) The treatment wasn’t a chemo drug, and it wasn’t a vaccine. Instead, doctors at the University of Pennsylvania had tried to make Walt’s own body the drug. In an approach known as gene therapy, they’d taken his own immune cells, modified them to give them new powers, and injected them back into his blood.

Gene therapy represents a break from the medical past. Like open-heart surgery, antibiotics and low-cost medical imaging, it’s a “disruptive” technology capable of changing the way doctors do business. It could transform how we treat many types of cancers in people of all ages—if it can be made to work. But that’s the problem. Before this trial at Penn—a Phase 1 trial, the earliest possible human test of a new treatment—gene therapy had scarcely worked in cancer, anywhere in the world. A typical gene-therapy experiment in cancer was as exciting as a sip of warm tea. Nothing happened, good or bad. In other kinds of gene-therapy trials, there had been tragedies: At Penn in 1999, in a trial run by doctors unrelated to the team treating Walt, a teenager with an inherited liver disease had died after a gene-therapy infusion sparked a runaway reaction.

But Walt’s doctors had done things differently than past scientists. Their approach was original and new. And, incredibly, they’d already succeeded in making tumors vanish in a few of the patients who’d come before Walt. Using their custom technology, the Penn physicians had jolted two cancer-riddled men into sudden apparent remission—an outcome dramatic enough to earn mentions on TV news and a write-up in the New York Times. In September 2011, the paper described Penn’s work as “a turning point in the long struggle to develop effective gene therapies against cancer.”

But now, eight months later, at the Hospital of the University of Pennsylvania, something dramatic was happening inside Walter Keller’s body—a riot of cells and signals. His blood pressure had crashed, so doctors had pumped him full of fluid to raise it, and the fluid had blown up his neck like a balloon. Socks were wrapped around his bloated legs to help with blood circulation. His kidneys were failing. He shook at times with “the rigors,” excruciating full-body shivers that made his whole body feel the way his heart would if he had just run up a huge hill.

Scientists don’t talk about “curing” cancer. A cure is the hope so great, so seemingly out of reach, that it must never be invoked. They’ve built a wall around the word. Still, the Penn researchers—as careful as they were, as professionally sober and skeptical—couldn’t help but wonder: Was their small experiment the start of something that could one day affect thousands, tens of thousands, more? Was it revealing a secret about the human body that could point the way to treatments for other cancers, not just leukemia? There was no way to know until they gathered more data. They needed to show that the therapy was safe. And they needed to prove that the early patients—the men whose tumors they’d blasted away—weren’t flukes.

Which is why so much now depended on Walter Keller. If Walt’s condition improved and his tumors diminished, the trial would move forward, and the potential of the Penn therapy—the result of a decades-long quest of scientific passion and discovery—would continue to grow. But if he suffered harm, Penn would have to pause the trial and maybe stop it altogether. Then everything would spiral down. Other scientists would argue that gene therapy was a dead end. Funding would dry up; research would wither. The Penn doctors might never get another chance to prove the merits of their idea, and we might all lose out. It had taken 20 years to get to this point, and it could all be over in the space of a few moments.

>>To read the rest of this story — including the team’s fight for funding, the tragic personal history of lead researcher Carl June, and what happened to Walt and his fellow patients — buy the e-book, Patient No. 7, available for e-readers, tablets, phones, and desktop computers at Amazon’s Kindle store.